A quotation attributed to Gilbert Adams in the Aug. 21 editions of The Washington Post was in error. Adams was quoted as saying he made a mistake in listing an apartment located in his single-family home on a District of Columbia real estate tax affidavit, resulting in a tax bill calculated at a higher commercial rate. What he actually said was that he probably should not have filed the affidavit.

Gilbert Adams lives in a house near Dupont Circle that looks like a single-family home and as far as he is concerned, is a single-family home.

So he was surprised and a bit angry recently when he got his District of Columbia property tax bill charging him the higher rate assessed against apartment houses. It cost him an extra $180.

The reason, Adams discovered by calling city hall, is that he rents a small apartment in the basement of his houses to a tenant. Under a tax law recently passed by the D.C. City Council, Adams' home is thus defined as a multiple-family structure and taxed as commercial property.

The problem could not have occurred last year, because all porperty in the city was taxed at the same rate, $1.83 for each $100 of assessed value.

But this year, as part of a tax reform package that cut taxes on most of the city's 100,000 single-family homes, a two-level tax structure was adopted.

The commercial rate was kept at $1.83 for each $100 of assessed value, while the rate for homes was dropped to $1.54.

Kenneth Back, director of the D.C. Department of Finance and Revenue, said the department is enforcing,to the letter, the definition of a single-family dwelling that the council wrote into its law setting the dual rates.

A single-family dwelling is defined as "improved residential property which is used exclusively for nontransient residential purposes and which contains not more than one dwelling unit, whether as a row, detached or semidetached structure." Condominiums and cooperative units also are included in the definition.

In its deliberations, the council did not discuss situations in which small apartments are distinctly secondary units in otherwise single-family homes.

Back said there are many units - he had no idea how many, but it could number hundreds or even thousands - that fall into the shadow area of the new law.

Does the law mean, for example, that any homeowner who rents a room to an outsider should have his dwelling taxed at the higher commercial rate?

"That's not before us," Back replied "My feeling would be that, if it is just a room, it would not be counted as another dwelling unit. But if you have an apartment with a separate entrance - like I understand Mr. Adams has - that would be multiple (unit). Mind you, I haven't made a (formal) ruling on this."

Back said part of the public confusion stems from the council enacting its real estate tax program in two separate laws.

The law that provides a home-owner's exemption reducing each dwelling's assessment by $9,000 applies to owner-occupied units in buildings tha contain up to five apartments.

To qualify for such an exemption, a homeowner had to file an affidavit with the France and Revenue Department by last April.

Adams said he filed his affidavit. "I think my mistake is, when I made it out, I listed the basement. If I hadn't informed them, I would automatically have gotten the residential $1.54 rate," Adams said.

Back said he hopes the situation will be repaired when the council acts on taxes next year.